The picture below is from the Plectronica intermission slideshow of pics of Bill's career. The pics are in chronological order, and this one is surrounded by pics of BBD playing live, following earlier pics of "Drastic Plastic" being recorded. So, assuming it's in the correct place, then it's from 1978.

If there was only size of stencil, then whatever Bill is sitting on is pretty close in size to your amp.

Always impressed by the historical knowledge on this forum. It's muddied the waters slightly - but in a good way. I think there may be some edge lines in the pic that aren't on my amp so the definitive still stands so far. Cheers again all.